The present invention relates generally to thermoplastic polyolefin compositions and, more particularly, to paintable thermoplastic polyolefin ("TPO") blends having a reduced flexural modulus and good paint retention while maintaining desirable impact resistance and tensile strength.
TPO generally refers to a wide range of olefin-based compositions or blends having many uses. Physical properties can be varied widely by adjusting the relative proportions of components.
Typical TPOs contain polypropylene, polyolefin rubbers,and optionally other ethylene copolymers. A particular example of a TPO comprises a melt blend or like mixture of a polypropylene resin with an ethylene-propylene rubbery polymer. The polypropylene resin imparts rigidity and temperature resistance to the TPO while the ethylene-propylene rubbery polymer imparts flexibility as well as toughness. Often, however, a lower flexural modulus is required than can be achieved without loss of other physical properties. TPOs such as those described above find particular application as rubber substitutes in the automotive industry in, for example, flexible interior and exterior body parts such as bumper covers, air dams, and other similar auto trim. These parts require a good combination of stiffness, impact resistance, and tensile strength. Some body parts require a lower stiffness than can be obtained without impairing other properties.
When used as an automotive exterior body part, paint should readily and durably adhere to the body part, so there is little visible difference between the painted part and the other painted portions of a vehicle. As a general rule, however, TPO compositions are difficult to paint, since most paints do not adhere well to ethylene and propylene based polymers.
A number of attempts have been made to externally modify these TPOs to improve their painting characteristics without sacrificing the important structural properties of the part. For example, the use of primers such as chlorinated polyolefins and electronic surface treatments have met with some success in solving the painting problem; however, these methods are also very expensive and time consuming.
An olefin high polymer modified by an ethylene/acrylic acid copolymer is described in U.S. Pat. No. 3,410,928 where the copolymer is used to improve adhesion, printability, grease resistance, and stress crack resistance. The copolymer is broadly defined to also include, among other things, terpolymers and multipolymers including many olefinically unsaturated esters, ethers, glycols, ketenes, and so forth. Acrylates and acrylic acids are also broadly disclosed as "copolymer" components, but there is no distinction made among the listed comonomers for physical property changes other than adhesion. The improvement of flexural modulus with good paint retention properties is not recognized. While molding is speculatively mentioned, there is no selection of comonomers and blend components to provide improvement in physical properties.
Also, a wide variety of blend components have been added to TPOs in an attempt to improve paint receptivity of the molded part. These blend components include, for example, various ethylene/acrylate and ethylene/unsaturated carboxylic acid copolymers used in combination with polypropylene and a copolymer of polypropylene as described in published PCT International Application WO86/04192, which is incorporated by reference herein for all purposes. While these components form generally satisfactory TPO's, they yield a higher flexural modulus than those compositions of the present invention as described below.